Buy this M31 Andromeda Galaxy photo.
High quality Robert Gendler astronomy picture. Photographs are available in a wide variety of sizes.Click to see selection as Astronomy Picture of the Day (APOD) - December 22, 2005

M31 has played a pivotal historical role in astronomy. Early observers saw the soft, foggy patch of glowing light as just another spiral nebula but weren't yet equipped with the knowledge to appreciate its nature. The true nature of M31 began to became clear in 1923. In that year Edwin Hubble, using the just completed 100 inch Hooker telescope at the Mount Wilson observatory, made his monumental discovery of Cepheid Variable stars in M31 and in one stroke forever changed the astronomical paradigm of the universe as we know it. Appropriately interpreting the cepheid data, Hubble was the first to appreciate the faint nebula in Andromeda as an "island universe", an immense galaxy in its own right, similar to our Milky Way. Hubble's work opened the door to the modern interpretation of the universe which we now know consists of countless galaxies all receding from each other. M31 has the distinction of being the nearest of all spirals at a distance of 2.5 million light years. Its disk, tilted toward earth by some 13 degrees, exposes the grandeur of its spiral structure and star systems to telescopic exploration.

M31, along with its near twin, the Milky Way, represent the two dominant giant galaxies of our Local Group which consists of some 40 members. Contrary to most galaxies which are receding away from each other, M31 and the Milky Way are actually moving toward each other and a close encounter or even a full collision may be in store for both galaxies in several billion years. Studies of globular clusters in M31 have revealed at least 4 different subpopulations including some much younger than those that exist in the Milky Way. These findings point to the strong possibility that the galaxy we know as M31 may have been formed by the cannibalization of numerous smaller galactic neighbors.